1. Field of the Invention
The present invention relates to an active belt steering control that minimizes lateral walk of the belt to within an allowable registration target tolerance. More particularly, it relates to a two position steering scheme yielding the advantages of absolute control.
2. Description of Related Art
The most effective means to steer a belt is by way of soft axis steering. That is, control or adjust the angle bisectrix axis of one of the rollers supporting the moving belt or web. Traditionally, two common control schemes are used for this steering: 1) absolute (continuously adjusted) soft axis angular position steering; and 2) two position (bang-bang) steering.
The absolute steering method controls the belt by adjusting the absolute soft axis tilt angle based on feedbacks of the belt lateral position and walk rate (which is the rate of lateral movement in one direction). The advantage of this method is precision which therefore make it suitable for multicolor printing or copying where registration of multiple passes is extremely critical. However, its disadvantages are the high cost of implementing an absolute soft axis tilt steering mechanism and the complication of potential nonlinearities in the design mechanics of the mechanism.
The two position steering method controls the belt by adjusting the soft axis tilt to either of two positions, one positive and one negative, based on the positional feedback of the belt as either left or right with respect to a fictitious plane. The advantages of the two position control are the simplicity and low cost of implementing a two position actuator mechanism. The disadvantage is that, heretofore, it has not provided the precision needed for color registration, and is only good for controlling the belt so that it will not walk away to one direction off the belt module.
U.S. Pat. No. 4,061,222 to Rushing discloses an apparatus for tracking an endless belt along an endless path by a tiltable belt steering roller whose position is continually adjusted so that the belt is maintained at a stable equilibrium position despite changes in the belt shape. The adjustment is determined by control circuitry which produces signals representative of lateral belt edge position, a desired belt edge position, and either a steering roller position or an instantaneous lateral belt deviation rate to produce a control signal which is applied to a gear motor to control the tilt angle of the steering belt roller. This apparatus utilizes the absolute control method.
U.S. Pat. No. 4,572,417 to Joseph et al discloses an apparatus for controlling lateral, cross track alignment of a web moving along a path to minimize lateral deviation between successive discrete areas of the web. A steering roller supports the web for movement along the path and is rotatable about an axis perpendicular to a plane of the span of the web approaching the steering roller.
U.S. Pat. No. 4,170,175 to Conlon, Jr. discloses a system for tracking an endless belt which automatically compensates for creep of the belt. The belt is supported by four rollers. A first is a drive roller, a second and third are idler rollers, and a fourth roller is an idler roller with flared ends. The flared roller provides passive tracking without electronic or active feedback. One of the idler rollers is spring loaded such that when an edge of the belt creeps up on one of the flared ends of the fourth roller, that side of the spring loaded roller is caused to tilt due to increased belt stiffness on that side. This positions the belt laterally toward a central position.
U.S. Pat. Nos. 4,174,171 and 4,344,693 to Hamaker disclose an apparatus for controlling the lateral alignment of a moving photoconductive belt. A resilient support constrains lateral movement of the belt causing a moment to be applied to a pivotably mounted steering post. As a result, the steering post pivots in a direction to restore the belt along a predetermined path. This apparatus is passive and provides no active electronic feedback.
U.S. Pat. No. 4,961,089 to Jamzadeh discloses a method and apparatus for controlling lateral movement of a web along an endless path. The lateral position of the web is monitored and a determination is made by a control unit if the web is within predetermined limits such that a copying operation can be completed while the web is still properly tracking. If the web is not tracking properly, or if it is predicted that the web will track beyond its predetermined lateral limits within a copying operation, a correcting step is taken prior to the copying operation. The correcting step determines a tilt angle for a steering roller. Upon completion of the correcting step, the apparatus returns to a monitoring capacity and does not provide corrective measures until the web is beyond or is predicted to go beyond the predetermined limits during a subsequent copying operation. This ensures that copying operations have proper registration and do not include corrective steps during the copying operation which might interfere with the registration. This apparatus uses an absolute scheme to determine corrective action.
In addition to electronic feedback active sensing of lateral position of the belt, some of the prior art described herein utilize mechanical elements which aid on controlling the belt such as resilient spring-loaded rollers and tapered rollers. While these reduce the cost of the apparatus, they are not of the precision to provide quality color registration.
Thus, the need exists for a low cost method and apparatus for controlling belt tracking which is precise enough to allow quality color registration, yet does not require complex steering mechanisms and controls.